Multiplate heat exchangers are generally formed by a leaktight chamber and by a plate pack placed in the said leaktight chamber and leaving a free space between them.
The plate pack is composed of a stack of mutually parallel plates leaving, between them, independent fluid flow circuits.
The plates, made of thin sheet metal, usually stainless steel or any other sufficiently ductile material, have edges with a smooth surface and a central part provided with corrugations by means of which they are in contact with one another and by means of which they delimit channels forming independent fluid flow circuits from one end of the heat exchanger to the other.
This type of heat exchanger with a plate pack operates with various fluids, in one-phase or two-phase mode.
If the charge to be heated is introduced into the exchanger in two-phase mode, it is necessary to mix the liquid and gas intimately so as to ensure uniform distribution of the mixture throughout the plate pack of the heat exchanger.
In order to ensure that the liquid and gas are intimately mixed, a device for injecting only the liquid phase is used.
This injection device consists of one or more nozzles which extend over the entire width of the plate pack and are provided with an open first end connected to pressurized-fluid feed means and with a closed second end.
The nozzle is generally placed in the lower part of the plate pack, and its axis is perpendicular to the channels of the said plate pack.
Moreover, the nozzle has calibrated holes for spraying the liquid into the channels of the plate pack, along generatrices and extending over the entire length of the nozzle.
The pressurized liquid enters the nozzle at one end and is sprayed upwards across the channels of the plate pack.
This pressurized liquid is then taken up by the gas, which flows at high speed, at the inlet of the plate pack, mixes intimately with the gas and flows, mixed with the gas, through the plate pack.
The two-phase mixture thus created is advantageously distributed over the entire width of the channels and is then entrained by the gas as far as the top of the plate pack.
This type of injection device perfectly fulfils the main function for which it was designed, namely, to ensure intimate mixing between the mixture and the gas and, consequently, to create the conditions for homogeneous and uniform distribution of the two-phase mixture as soon as it enters the plate pack.
A major drawback resides in the fact that the calibrated holes in the injection nozzles may be partially or completely blocked because of the presence in the liquid of particles or debris, for example metal particles or debris, despite the presence of filters inside the feed pipe for the liquid.
Thus, the operation of the heat exchanger is disturbed and its performance gradually deteriorates.
The injection nozzles must therefore be able to be cleaned regularly and, to accomplish this, the entire plant is shut down in order to be able to remove these injection nozzles and to clean them.
The operations of dismantling and cleaning the injection nozzles, and of refitting them, require, on average, shutting down the plant for one week.